Perhaps one of the most impressive features of the iPhone 4, the Retina display, was introduced at yesterday’s Apple Worldwide Developer Conference yesterday.

To answer the question of what a Retina display is, Chris Branderick of PC World offered the following tidbits:

“To put it simply, Apple’s figurative Retina display is an LCD that boasts a super high pixel density by squeezing a 960-by-640-pixel resolution into 3.5 inches—a pixel density of 326 pixels per inch (ppi).

The Retina display has four times the number of pixels as previous iPhones; its screen size is unchanged, resulting in double the pixel density. When compared to the now US$99 iPhone 3GS, which has a 163ppi screen with a 480-by-320 resolution, it’s easy to imagine just how this new screen will shine.

While talking about the new display, Steve Jobs went on to detail that after a certain point the human eye fails to distinguish individual pixels. According to the Apple CEO this “magic number,” when visible pixelation is no more, is around 300ppi. Therefore, with the iPhone 4’s screen coming in at more than 300 pixels per inch (326ppi), the display will supposedly always looks smooth and crisp, with no jaggies in sight.

Apple’s retina display also promises an improved contrast ratio. The company claims that the upcoming iPhone 4 will have a contrast ratio four times higher than that of previous models. Beyond the use of more compact pixels the screen, which is a backlit LED, will also adopt In-Plane Switching (IPS) technology to improve viewing angles and enhance color display.

Apple isn’t the first to put a super-high-resolution screen in a smartphone. Google’s Nexus One, for example, features an OLED screen with a resolution of 800-by-480 pixels, but its subpixel arrangement has some issues.”

Stay tuned for additional details as they become available and if you’re attending the WWDC and can offer any hands-on feedback of the iPhone 4, please let us know in the comments.